High-temperature, light-emitting devices, such as incandescent light bulbs are conventionally coated for translucency with liquid coatings comprising silicone resins in organic solvents. There is a general need to replace such hazardous and polluting coatings with non-hazardous and non-polluting powder coatings.
Powder coating compositions which yield transparent or translucent coatings are well known in the art. Typical organic binder materials for such coatings are epoxy, polyester, and acrylic resins. Unfortunately, coatings based on these organic binder systems darken and decompose upon prolonged exposure at typical incandescent operating temperatures of 300 to 700.degree. F. (149 to 371.degree. C.) and thus are not useful for light-emitting devices. Coatings containing blends of silicones and significant levels of organic binders, such as those disclosed in U.S. Pat. No. 5,684,066 to Eklund and U.S. Pat. No. 4,877,837 to Reisling also discolor and similarly are not useful for light-emitting devices.
Coating powders based totally on silicone resins or substantially entirely on silicone resins are known. For example, Daly et al. in U.S. Pat. No. 5,422,396 disclose in a "comparative example" a 100% silicone resin based on Dow Corning 6-2230 silicone resin. The "comparative example" formulation in Daley et al. contained 80 phr (parts per hundred resin by weight) mica. Although silicone-based powder coatings having as low as 40 phr mica or other reinforcing filler have been described, higher levels such as 60 phr and upward are generally used in high-temperature coatings. Below about 60 phr filler levels, coatings tend to be insufficiently reinforced for high-temperature use as such coatings tend to crack and peal at high temperatures.
Silicone coatings containing 60 to 80 phr and upward filler, such as mica and/or wollastonite (calcium metasilicate) are resistant to cracking and peeling at high temperatures. However, such fillers contribute to opacity and yellowing of the coatings, and filler levels of 60 phr give coatings which are too rough and too opaque to be generally useful for light-emitting devices.
It is a general object of the invention to provide coating powders for producing translucent coatings on high-temperature substrates, such as light-emitting devices including incandescent bulbs.